The present invention relates to cargo transfer, more particularly to cargo transfer involving marine vessels at sea.
A Landing Craft Air Cushion (LCAC) is a high-speed air-cushion hovercraft. The LCAC is typically used by the U.S. Navy as an amphibious landing craft for carrying weapons, equipment, cargo and personnel from a ship (e.g., an amphibious assault ship) to the shore, and for returning to the ship for another load. Current U.S. Navy Sea Base plans call for a capability to launch and support the operations of a Marine Expeditionary Brigade (MEB) from the ships of the Sea Base. The LCAC is the primary surface assault connector. The LCAC has the ability to climb and descend slopes; however, the U.S. Navy's need to fully exploit these climbing/descending capabilities did not arise until presently, as the LCAC is the prime surface assault connector of the Sea Base.
Unfortunately, there is no existing asset to bring the necessary number of required LCACs into theater. Another problem is how to load these LCACs to support the MEB in an efficient and timely manner. Current methods of loading LCACs from larger cargo ships at sea involve loading LCACs while they are in the water, or driving them onto lightweight temporary platforms that are relatively small in size and hence subject to substantial motion as sea states rise. It simply takes too much time to load each LCAC using current methods. Alternative approaches have been suggested that would use a ship as both an LCAC carrier and a transfer enabler for the Sea Base. Some of these concepts require the ship to ballast-down, as in a heavy lift ship, so that the LCACs can fly on and off the mother ship. Other concepts use large elevators to transfer the LCAC between the carrier and the water. These concepts are complex and create a critical failure path.
It is also desirable for two or more ships to have the capability to moor together while at sea. However, the forces creating the relative vertical motions between two or more ships are too powerful to be overcome by traditional mooring and tendering systems. To fight these forces would mean fighting the entire restorative buoyancy force. Aside from welding the ships together (certainly a preposterous solution), this is virtually unachievable. Analysis shows that in Sea State 4, the upper requirement for Sea Base operations, the relative vertical movement between two ships moored together will be too great to allow the safe transfer of personnel and cargo.
Of particular interest herein is the capability to transfer cargo and personnel between two different marine vessels at sea. For the onloading and offloading of cargo, traditional “Roll-On/Roll-Off” (RO/RO) ramps operate through the bow or stern of a ship, and in association with a pier. According to this conventional practice of cargo transfer between a ship and a pier, the height of the pier is known either as a specific value or as falling within a specific range of values. In contrast, for ships at sea, it would be extremely difficult to moor two or more ships bow to stern so that an RO/RO procedure could occur. Nevertheless, in certain U.S. Naval evolutions (such as involving cargo/personnel transfer a Sea Base), a large number of vehicles must be transferred from one ship to another. While a crane could be used to move these vehicles, an RO/RO operation would be much more efficient insofar as the vehicles would, in effect, be moving themselves between the ships. The RO/RO concept in general principle is advantageous in that it avails itself of the inherent ability of transportation vehicles to move from one place to another.
Since it is impracticable that ships at sea moor bow-to-stern, the possibility suggests itself that some sort of RO/RO methodology be performed transversely between two ships at sea that are moored “skin-to-skin.” However, this introduces another problem in that the freeboard between ships can vary quite a bit, and ships with side-ports offer an even lower access point. It has been believed to be impossible to develop a multi-purpose ramp system (i.e., a ramp system that is not ship-specific) that could accommodate the wide range of potential vertical heights of the various ships that may require transfer, on and/or off, of vehicles and/or personnel.